Use of horizontal wells in oil reservoirs is currently of high interest within the oil industry. Horizontal wells allow more reservoir surface area to be contacted and thereby reduce inflow pressure gradients for reasonable oil production rates. Alternatively, for typical pressure gradients within the wellbore region, the productivity of a horizontal well is greater than that in a vertical well.
Possible benefits of horizontal wells are currently being exploited in the Canadian tar sands. Reservoirs in Canada that may be categorized as immobile under reservoir conditions include the Cold Lake and Athabasca deposits. Current practices for producing the above immobile tar sands include mining and solvent stimulation. Solvent stimulation is also used to remove very viscous oils from formations or reservoirs.
U.S. Pat. No. 4,373,585 issued to Fitch et al. discloses a method of recovering viscous oil from a viscous oil-containing formation wherein a selected solvent is injected into a fluid communication path in the lower portion of the formation intermediate between an injection well and a production well. A hydrocarbon solvent having a density less than oil contained in the formation and a viscosity not greater than 1/100 the viscosity of the oil contained in the formation under formation conditions is injected into the communication path. Fluids including oil are recovered from the production well until recovered fluid contains an unfavorable ratio of oil to solvent. The production well is shut-in and an additional quantity of the hydrocarbon solvent is injected into the fluid communication path.
Subsequently, the production well is also shut-in to permit the formation to undergo a soak period for a variable time. A driving fluid is then injected into the formation via the injection well and the oil is produced until there is an unfavorable ratio of oil to driving fluid. During the fluid drive recovery phase, the injection well and production well may be completed to be in fluid communication with the entire portion of the formation to obtain a more uniform displacement of the solvent and oil mixture in the formation by the driving fluid.
U.S. Pat. No. 4,293,035 issued to Fitch discloses a method of recovering viscous oil from a viscous oil bearing subsurface formation wherein a solvent is injected into a high mobility channel formed in the bottom of the formation intermediate an injection well and a production well. The solvent is injected until the ratio of produced oil to solvent becomes unfavorable. Thereafter, the injection of solvent is terminated and gas is injected into the high mobility channel to produce solvent and oil from the formation.
In U.S. Pat. No. 3,838,738 there is described a method for recovering viscous petroleum from petroleum-containing formations by first establishing a fluid communication path low in the formation. A heated fluid is then injected into the fluid communication path followed by injecting a volatile solvent such as carbon disulfide, benzene or toluene into the preheated flow path and continuing injecting the heated fluid and recovering fluids including petroleum from the production well.
In U.S. Pat. No. 3,500,917 there is disclosed a method for recovering crude oil from an oil-bearing formation having a water-saturated zone underlying the oil-saturated zone. A mixture of an aqueous fluid which has a density greater than the density of the crude oil and a solvent having a density less than the density of the crude oil are injected into the water-saturated zone and oil is produced from the formation.
U.S. Pat. No. 4,026,358 discloses a method for recovering heavy oil from a subterranean hydrocarbon-bearing formation traversed by at least one injection well and one production well wherein a slug of hydrocarbon solvent in amounts of 0.1 to about 20 percent of the formation pore volume and having a gas dissolved therein is injected into the formation via the injection well. Thereafter, a thermal sink is created in the formation by in-situ combustion or by injecting steam. The wells are then shut-in for a predetermined time to permit the formation to undergo a soak period, after which production is continued. Optionally, after the production period, the formation may be water flooded to recover additional oil from the formation.
Butler et al. in U.S. Patent No. 4,116,275 issued Sep. 26, 1978, teach a cyclic steam stimulation method for removing viscous fluids from a formation penetrated by a horizontal wellbore. Said wellbore contains a perforated casing and dual concentric tubing strings.
Solvents have a beneficial result since they dilute the crude, thereby making it mobile due to the reduction in viscosity. However, their use has not been practical commercially since this process evolves long periods of soak-time to allow the solvent to mix with the crude. Therefore, the critical factor is the soak time needed, and depending on the thickness of the oil zone, the soak time may vary from a year or two up to possibly eight or more years.
Therefore, what is needed is a solvent stimulation method for removing hydrocarbonaceous fluids from immobile tar sands or viscous fluids via a horizontal wellbore which will avoid long soak-times while providing for simultaneous solvent stimulation and continuous hydrocarbonaceous fluid production.